Patent Literature 1 proposes a lateral MOSFET having high breakdown voltage and high reliability. This lateral MOSFET includes a conductive field plate disposed on a thick insulation film on both the source side and drain side with a gate interposed therebetween. The field plate is made of Poly-Si, for example, and has a floating potential not electrically connected with other electrodes or the like.
According to the structure including this field plate, the potential of the field plate is set to an intermediate potential between potentials of a gate electrode and a drain layer when the potential difference between the gate electrode and the drain layer increases. Accordingly, each of the potential difference between the gate electrode and the field plate, and the potential difference between the field plate and the drain layer is kept smaller than the potential difference between the gate electrode and the drain electrode, in which condition electric field concentration below the thick insulation film decreases. As a result, the breakdown voltage increases.
There is also a structure which provides a resistant field plate between the high potential side and the low potential side of a lateral IGBT to decrease a potential in the direction from the high potential side to the low potential side. This field plate has a spiral shape, for example. One and the other ends of the field plate are connected with the high potential portion and the low potential portion, respectively. The structure including this field plate produces a gradual potential change even in a potential distribution below the field plate, thereby reducing electric field concentration for prevention of breakdown caused by electric field concentration, and reduction of a loss produced at the time of switching.
However, when the field plate has a floating potential as in Patent Literature 1, charges are easily produced within Poly-Si constituting the field plate, or the interface between the insulation film and Poly-Si. These charges cause on-resistance fluctuations. According to simulation conducted by using an estimated model of a lateral MOSFET illustrated in FIG. 11, it has been confirmed that movable ions contained in an interlayer insulation film of a BPSG (Boro-phospho silicate glass) film 1 are accumulated in an LOCOS oxide film 32, and affect a current path. Specifically, as illustrated in FIG. 11, the current path is formed at a position deeper than the interface between the LOCOS oxide film J2 and a drift layer J3 by negative charges of the movable ions accumulated in the LOCOS oxide film J2. In this condition, sufficient reduction of on-resistance fluctuations is difficult to achieve.
On the other hand, in case of the structure connecting one and the other ends of the field plate with the high potential portion and the low potential portion, respectively, a current of 1 μA or higher flows in the field plate based on the potential difference between the ends of the field plate. As a result, a current loss increases.